An excellent shut-off performance is required of a shut-off valve used in a gas fired boiler, a gas combustion apparatus or the like. As a conventional shut-off valve in which an improvement in shut-off performance has been achieved, the following shut-off valve is known. In this shut-off valve, a valve seat is provided in a flow passage of a valve casing equipped with a fluid inlet and a fluid outlet, and a valve body, which is connected to an electromagnetic valve opening means and which is supported by the forward end of a valve operating shaft adapted to move axially, is urged by a compression spring to be brought into contact with the valve seat, and is caused to move away from the valve seat by the electromagnetic valve opening means. The valve seat is composed of a first valve seat portion formed on the inlet side with respect to a valve hole as a center, and a second valve seat portion formed on the outlet side and of a smaller diameter than the first valve seat portion. Further, the valve body is composed of a first valve body portion of a large diameter in contact with the first valve seat portion, and a second valve body portion of a small diameter in contact with the second valve seat portion, with the first and second valve seat portions being superimposed one upon the other to be integrated with each other (see JP 2003-161375 A).
In the above-described shut-off valve, when opening the valve, the valve body is separated from the valve seat against the elastic force of the compression spring by the attracting force of the electromagnetic valve opening means to thereby open the valve hole. At this time, the fluid pressure in the fluid passage acts on the valve body, so that the attracting force of the electromagnetic valve opening means is required to be strong enough to overcome the elastic force of the compression spring and the fluid pressure in the flow passage. The fluid pressure on the valve body is in proportion to the pressure reception area of the valve body, so that the larger the diameter of the valve body, the larger the fluid pressure on the valve body, and the larger the electromagnetic valve opening means, resulting in an increase in cost.
In the above-described conventional shut-off valve, the fluid pressure acts on the first valve body portion of a large diameter on the fluid inlet side, so that it is necessary to provide an electromagnetic valve opening means having an attracting force stronger than the fluid pressure acting on the first valve body portion of a large diameter at the time of valve opening. Thus, when compared with a valve body composed of a single valve body portion, it involves an increase in the size of the electromagnetic valve opening means, resulting in an increase in cost.